Soap and the Spanish for the same, jabón, sound like they have nothing in common. But sounds can be deceiving.
Both come from the same root: the Latin sebum, meaning “grease”.
How can such different words be so related? Easy: the Latin s- sound and its variations (sh‑, ch- and sy- for example) usually became, under the arabic influence, a j- sound in Spanish but remained more intact in English.
Thus, the s‑p of soap maps almost exactly to the j‑b of jabón. The “p” and “b” are often easily interchanged as well.
Less fun is also noting that, from the same Latin root, meaning “grease” we also get seborrhea (a medical condition of having too much grease on your skin).
The Spanish caja (“box”) comes from the Latin capsa for the same.
This gives us a surprising connection to some English words that, on the surface, sound very different than caja:
The Latin turned into the Spanish through an interesting pattern: the ‑sh- sound in Latin consistently turned into the ‑j- sound in Spanish (at first retaining the original pronunciation, but then under the influence of Arabic, grew to the throat-clearing sound). With caja, we have a slight variation of the pattern, where the ‑ps- sound turned into the ‑j- sound. Thus, the c‑ps maps exactly to c‑j.
The Latin sounds for “sh” — and similar variations, like “ch” and “ss” — became a “j” sound in Spanish.
Thus, the English sherry is near identical to the Spanish jerez!
These sh/j sounds were often spelt with a “x” in old Spanish; and sherry itself is named after the town it first came from, Xeres, which is near Cordova.
Chief, and the Spanish for the same, Jefe, both come from the same root: the French chef, which means the same.
But this is odd as they sound so different! How are they related?
It’s not obvious, but it’s easy once you understand the pattern: The Latin sound “sh” and very similar sounds (such as the “ch” and “sy”) almost always became a “j” in Spanish. Like syrup and jarabe. Not obvious!
The Spanish flojo means “slack, loose” — but it is a very common word in Spanish, often used to mean “relaxed” in a negative way, in senses like, “They cut themselves some slack.”
Flojo comes from the Latin fluxus, meaning the same as the Spanish. From fluxus, we get a bunch of English words, including: fluent, fluid, fluctuate and even (via fluent) affluent and influence. We also get the more fun flush and the most obvious flux (as in, “to be in flux.”) All of these can be understood in the sense that, that which is loose flows — and all of these words flow in one way or another: liquids are fluid, you speak fluently, flushing water flows, money flows if you are affluent, etc.
The ‑x- in the original Latin tended to disappear into the English (hence leaving the vowels before and after, as in fluent or fluid) or became a ‑sh- sound. This is an example of the common pattern of the ‑sh- sounds mapping to the throat-clearing ‑j- in Spanish, with the fl-sh of flush mapping to the fl‑j of flojo.